1. Field of Invention
The present invention relates generally to the field of nuclear magnetic resonance (NMR), and more specifically to an NMR probe wherein the probe itself constitutes a transmission line.
2. Prior Art
The nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B0, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B0, the aligned magnetic moments precess about the axis of the field at a frequency which is dependent on both the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precessional frequency, ω, also referred to as the Larmor frequency, is given by the Larmor equation ω=γB, in which γ is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B0 plus other fields) acting upon the nuclear spins. It is thus apparent that the resonant frequency is dependent on both the nuclei contained in the sample as well as the strength of the magnetic field in which the sample is placed.
The orientation of magnetization M, normally directed along the magnetic field B0, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields, designated B1, are applied orthogonal to the direction of the B0 field by means of RF pulses through a coil connected to radio-frequency-transmitting apparatus. Under the influence of RF excitation, magnetization M rotates about the direction of the B1 field. In NMR studies, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B0 field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane precess around the direction of the static field. The vector sum of the spins forms a precessing bulk magnetization which can be sensed by an RF coil. The signals sensed by the RF coil, termed NMR signals, are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. As is evident from the Larmor equation, the frequency of these signals, and thus the frequency at which the RF coil or coils in an NMR probe must transmit and receive, is dependent on the magnetic field and the nuclear species.
Conventional NMR probes employ a resonant circuit which must be tuned near the frequency of interest. The bandwidth of frequencies to which a conventional probe is sensitive is thus limited by the Q of the resonant circuit. This limits the range of magnetic fields and/or nuclear species to which a conventional probe is sensitive without changing the frequency to which the resonant circuit is tuned. In many applications of NMR this bandwidth limitation poses substantial difficulties. It is particularly problematic in studies where it is desirable to vary the B0 field over a wide range of magnitudes, in studies of quadrupolar nuclei (which contain very broad resonances), in studies of multiple nuclear species, or whenever physical access to the tuning elements of the probe is limited. It will be thus apparent that in many applications it is desirable to have an NMR probe which is not limited in bandwidth. It will be thus further apparent that the limitation in bandwidth is related to the presence of a resonant circuit.
Another important aspect of NMR probes is an impedance matching circuit. Because NMR probes transmit and receive RF signals, it is important to match the probe impedance to the impedance of the apparatus to which it is attached (usually an NMR spectrometer). In a probe based on a resonant circuit, the impedance depends on the frequency, so an additional electronic circuit called a matching circuit is required. This circuit, frequently based on tunable reactances, is adjusted until the probe impedance matches the spectrometer impedance. Because changing the resonant frequency of a tuned probe changes its impedance as well, any time the tuning of a resonant circuit probe is adjusted the matching network must be adjusted as well. This is often a time consuming process. Thus it is desirable to have an NMR probe with an impedance which does not depend on frequency. Such a probe is particularly desirable in applications such as those described above, where the frequency of interest is varied substantially.
A further characteristic important in the behavior of NMR probes is coil geometry. Two types of coil which have traditionally been used for NMR studies of an object are the simple solenoid and the saddle coil. Which coil is used will typically depend on the geometry of the structure which provides the main static magnetic field. For example, if the geometry only permits the object under study to be inserted in a direction perpendicular to the lines of flux of the main magnetic field, a solenoidal coil is most efficient. This is the case when the main magnetic field is provided by a resistive or a permanent magnet. On the other hand, when the object under study is inserted in the same direction as the lines of flux, as in a superconducting magnet with a horizontal bore, the saddle coil must be used. Both types of coil are discussed by D. Hoult and R. Richards, “The Signal-to-Noise Ratio of the Nuclear Magnetic Resonance Experiment”, Journal of Magnetic Resonance, Volume 24 (1976), p. 71-85. An example of a saddle coil can be found in U.S. Pat. No. 4,398,149. Because it is based on a resonant circuit, it suffers all of the bandwidth limitations inherent to conventional tuned probes.
Because horizontal bore superconducting magnets favor the use of saddle coil probe geometries, it is thus apparent that in many applications it is desirable to have an NMR probe of the saddle coil geometry which is not limited in bandwidth.
Several attempts have been made to overcome the limitations in bandwidth imposed by resonant tuned circuits in NMR probes. I. J. Lowe and M. Engelsberg, “A Fast Recovery Pulse Nuclear Magnetic Resonance Sample Probe Using a Delay Line”, Review of Scientific Instruments, Vol. 45, No. 5, May 1974, pp. 631-639, disclosed a lumped parameter delay line, a design which was modified in I. J. Lowe and D. W. Whitson, “Homogeneous RF Field Delay Line Probe for Pulsed Nuclear Magnetic Resonance”, Review of Scientific Instruments, Vol. 48, No. 3, March 1977, pp. 268-274. These probes are delay lines, however, thus have an intrinsic cutoff frequency. In addition they are difficult to manufacture. Furthermore, they are of geometries inappropriate for superconducting magnets with horizontal bores. They also present an impedance which depends on frequency. Atsushi Kubo and Shinji Ichikawa, “Ultra-broadband NMR probe: numerical and experimental study of transmission line NMR probe”, Journal of Magnetic Resonance, Vol. 162, Issue 2, June 2003, pp. 284-299, also use lumped elements to construct delay line probes which have intrinsic cutoff frequencies. These probes are also not of the saddle coil geometry, and in addition suffer a frequency dependent impedance. Furthermore, their electrical behavior is cyclic as a function of frequency rather than uniform.
If a probe is constructed such that the coil itself constitutes a transmission line, and that transmission line is properly terminated, then the coil will be substantially sensitive to a wide range of frequencies. This is because in a loss-free transmission line, the impedance is independent of frequency. Thus signals of all frequencies in such a line propagate with the same facility. The impedance of such a transmission line is given by Z=√{square root over (L/C)}, where L and C are the inductance and capacitance per unit length. Transmission lines are discussed by D. M. Pozar, “Microwave Engineering”, 3rd Ed., John Wiley & Sons, 2004.
As a point of clarification, it should be noted that a probe constructed such that the probe itself constitutes a transmission line is distinct from the situation in which an NMR probe contains a transmission line. Many NMR probe designs include transmission line segments in order to move tuning and matching elements farther from the coil, or to aid in impedance matching a particular portion of the coil, or in other functions of design. However, these designs are still based on resonant circuits, thus suffer the penalties of limited bandwidth, stringent tuning, and frequency-dependent-impedance-matching requirements associated with resonant NMR probes.